1. Field of the Invention
The present invention is directed to a method for the operation of a computed tomography (CT) apparatus with which volume data for a volume region of an examination subject can be registered.
2. Description of the Prior Art
Before the definition of a region for which volume data are to be registered, for example with a spiral scan, an x-ray shadowgram (topogram) is usually registered, with reference to which the scan region of the following spiral scan is graphically defined. The definition of a scan region ensues by graphic marking of a generally rectangular region in the x-ray shadowgram that encompasses the region of interest of the examination subject. The length of the rectangle defines the length of the spiral scan; the width of the rectangle defines the width of the field of view shown in the CT image.
In the case of multiple spiral scans, the above procedure is repeatedly applied for defining each spiral scan.
An object of the present invention is to provide a method of the type described above that makes it easy for an operator to acquire the respectively desired, diagnostic information.
The above object is achieved in a method for operating a computed tomography apparatus in accordance with the principles of the present invention, wherein volume data for a volume region of an examination subject are registered, wherein an x-ray shadowgram of a region of an examination subject containing the volume region is produced, and a plurality of markings are mixed into the x-ray shadowgram for identifying reconstruction regions in the volume region for which image data are to be reconstructed from the volume data, and wherein at least one reconstruction parameter is allocated to each of the reconstruction regions, and wherein image data are reconstructed for each of the reconstruction regions employing the (at least one) reconstruction parameter allocated to the respective reconstruction region.
It is thus possible to mark a number of regions within, for example, the volume data registered in a spiral scan, from which a number of regions a reconstruction of image data then ensues with reconstruction parameters that are appropriate for the respective region of the examination subject, for example the slice thickness underlying the reconstruction, referred to as the thick reconstructed slice, the convolution kernel to be applied in the reconstruction, etc.
The invention thus facilitates the clinical use of CT apparatuses, particularly in applications, on the basis of a facilitated operation and optimized executive sequence, for which it is necessary to reconstruct sub-sections of an organ, particularly for the diagnosis of, for example, an organ with a different slice thickness than other subsections of the same organ. This is because, in accordance with the invention, the reconstruction ensues on the basis of volume data acquired during a single spiral scan, with marking of the reconstruction regions simultaneously ensuing in a single work step and in a single x-ray shadowgram.
In a version of the invention, the data required for the production of the x-ray shadowgram are registered before the registration of the volume data, and the registration of the volume data ensues after the identification of the reconstruction regions, so that volume data with respect to each reconstruction region are available. The marking of the reconstruction regions thus ensues prospectively before the registration of the volume data.
Alternatively, in another version of the invention the marking of reconstruction regions ensues for volume data already registered, on the basis of an x-ray shadowgram belonging to the volume data already registered; and the reconstruction of image data corresponding to the reconstruction regions ensues based on the volume data already registered. This means that a retrospective definition of reconstruction regions is also possible insofar as volume data are present. If a shadowgram belonging to the volume data is not already present, this can be produced by acquiring the data required for the production of the x-ray shadowgram from the volume data that have already been registered.
In a preferred embodiment of the invention, the volume data are registered in the form of a spiral scan.
In a further embodiment of the invention, when a number of reconstruction regions are marked, these can at least partially overlap one another. This offers the advantage that regions of the examination subject contained in a number of reconstruction regions are not multiply scanned and thereby need not be charged with x-radiation.
In another embodiment of the invention, at least the slice thickness and/or convolution kernel can be prescribed as reconstruction parameters.
A modern multi-slice CT apparatus, i.e. a CT apparatus with a detector system formed by a number of lines of detector elements, is able to scan volumes with high axial resolution, i.e. narrow collimation (small slice thickness of the slices of the examination subject scanned with the individual lines of the detector system) in a single spiral scan. Volume data result from this scan from which, for example, images of thin or thick slices can be subsequently reconstructed. Thus, it is possible for the user to acquire different diagnostic information from volume data registered in a single spiral scan with tight collimation: thinner slices in order to acquire information about highcontrast structures, for example bones, vessels filled with contrast agent, air-containing bronchia or prepared intestinal material, and thicker slices in order to be able to acquire information about low-contrast structures, for example soft tissue parts.
A typical example of a spiral scan is a scan of the skull with a collimation of 4*1 mm. The radiologist needs slice thicknesses of 3 mm or 4 mm for the base of the skull; slice thicknesses of 5 mm through 8 mm are standard for the cerebellum. Given simultaneous CTA (CT angiography), the thinnest slices of 1 mm are required, for example for the presentation of the circulus Willisis (Circle of Willis).
Similar demands arise in the examination of other organs such as the lung, with high-resolution images having a 1 mm slice thickness, and standard images having a 5 mm slice thickness or CTA of the abdomen or examination of the entire aorta with the various arterial branchings.
Depending on the clinical demands, the reconstruction ranges can overlap to form a spiral range or can be nested inside one another.
When, as in a multi-phase liver examination, a number of spiral scans are implemented, method can be employed in the aforementioned way with respect to each and every individual spiral scan.